The present invention relates to method of making a photographic imaged element having a protective overcoat that resists fingerprints, common stains, and spills. More particularly, the present invention involves a processing-solution-permeable overcoat in the manufactured photographic (imaging) element that becomes water resistant in the finished (imaged) product. The overcoat formulation comprises at least one water-dispersible hydrophobic polymer interspersed with a water-soluble polymer. The incorporation of a small amount of a water-soluble organic solvent in the last step of the photographic process, i.e., during the washing step, induces the formation of a durable, water-resistant film.
Silver halide photographic elements contain light sensitive silver halide in a hydrophilic emulsion. An image is formed in the element by exposing the silver halide to light, or to other actinic radiation, and developing the exposed silver halide to reduce it to elemental silver. In color photographic elements, a dye image is formed as a consequence of silver halide development by one of several different processes. The most common process is to allow a by-product of silver halide development, oxidized silver halide developing agent, to react with a dye-forming compound called a coupler. The silver and unreacted silver halide are then removed from the photographic element, leaving a dye image. Formation of the image commonly involves liquid processing with aqueous solutions containing a developing agent that must penetrate the surface of the element in order to come into contact with silver halide and coupler. Thus, gelatin or similar natural or synthetic hydrophilic polymers have proven to be the binders of choice for silver-halide photographic elements.
Unfortunately, when gelatin or similar polymers are formulated so as to facilitate contact between the silver-halide crystals in the photographic element and aqueous processing solutions, the resulting product lacks fingerprint and stain resistance. This is problematic in view of the handling and environmental exposure that photographic images commonly experience under various circumstances. Thus, for example, fingerprints can permanently mark the image, or common household products, such as foods or beverages, coffee spills, mustard, and the like can easily stain the image.
There have been attempts over the years to provide protective layers for gelatin based photographic systems that will protect the images from damages by water or aqueous solutions. For example, various lamination techniques are known and practiced in the trade. U.S. Pat. Nos. 3,397,980, 3,697,277 and 4,999,266 describe methods of laminating a polymeric sheet film, as a protective layer, on a processed image. Protective coatings that need to be applied to the image after it is formed, however, adds a significant cost to the final imaged product.
A number of patents have been directed to water-resistant protective coatings that can be applied to a photographic element prior to development. For example, U.S. Pat. No. 2,706,686 describes the formation of a lacquer finish for photographic emulsions, with the aim of providing waterxe2x80x94and fingerprint-resistance by coating the light-sensitive layer, prior to exposure, with a porous layer that has a high degree of water permeability to the processing solutions. After processing, the lacquer layer is fused and coalesced into a continuous, impervious coating. The porous layer is achieved by coating a mixture of a lacquer and a solid removable extender (ammonium carbonate), and removing the extender by sublimation or dissolution during processing.
U.S. Pat. No. 5,856,051 describes the use of hydrophobic particles with gelatin as the binder in an overcoat formulation. This invention demonstrated an aqueous coatable, water-resistant protective overcoat that can be incorporated into the photographic product, allows for appropriate diffusion of photographic processing solutions, and does not require a coating operation after exposure and processing. The hydrophobic polymers exemplified in U.S. Pat. No. 5,856,051 include polyethylene have a melting temperature (Tm) of 55 to 200xc2x0 C., and are therefore capable of forming a water-resistant layer by fusing the layer at a temperature higher than the Tm of the polymer after the sample has been processed to generate the image. The coating solution is aqueous and can be incorporated in the manufacturing coating operation without any equipment modification. Again, however, fusing is required by the photofinishing laboratories to render the protective overcoat water-resistant. Similarly, commonly assigned U.S. Ser. No. 09/353,939 and U.S. Ser. No. 09/548,514, respectively, describe the use of a polystyrene-based material and a polyurethane-based material, with gelatin as the binder, in an overcoat for a photographic element, which overcoat can be fused into a water resistant overcoat after photographic processing is accomplished to generate an image.
Commonly assigned U.S. Ser. No. 09/235,436 discloses the use of a processing solution permeable overcoat that is composed of a urethane-vinyl copolymer having acid functionalities. Commonly assigned U.S. Ser. No. 09/235,437 and U.S. Ser. No. 09/448,213 disclose the use of a second polymer such as a water-soluble gelatin or polyvinyl alcohol to improve processibility and reduce coating defects.
It has already been shown that protection of an imaged photographic element, for example a photographic print or film, can be achieved by applying a top (overcoat) layer to the photographic element during manufacture, which overcoat comprises a film-forming hydrophobic polymeric component dispersed together with a water-soluble, relatively low molecular weight polymer. During processing, the low molecular weight, water-soluble polymeric material is washed out of the coating, so that when the processed film is dried, the remaining hydrophobic polymer forms a durable protective film. However, not all hydrophobic materials work equally well for this application. For example, not all of them can form a film after processing or a film that is as water resistant as desirable, particularly at the relatively low temperatures that are typically used for drying the processed element. While some of these materials can be made to form such a film by a high temperature fusing step, the addition of this step to the process is inconvenient and expensive. Also, the type of materials that can be made to coalesce to form a film under typical photoprocessing conditions tend to be less durable than coating materials that require fusing.
It would be desirable provide an improved protective overcoat that is convenient and economical. It would also be desirable to obtain a water-resistant protective overcoat for a photographically imaged element without requiring the addition of laminating or fusing steps, without the need for high temperatures, and without requiring additional equipment to carry out photoprocessing or finishing.
The invention relates to the conversion of a process-permeable overcoat (xe2x80x9cprecursor protective layerxe2x80x9d) on a photographic element to a durable, waterxe2x80x94and stain-resistant protective overcoat. In particular, the present method comprises the incorporation of a small amount of a water-soluble organic solvent in the last step of the photographic process, i.e., during the final washing step. Applicants have found that some polymers that are not useful or not ideal for making a durable overcoat layer in the normal process because incomplete or unsatisfactory film formation can be induced to form a durable, water-resistant film if washed in the presence of a water-soluble organic solvent. Thus, this invention substantially increases the scope of the materials that are useful for forming a process-permeable overcoat that is convertible to a water-resistant protective overcoat. In particular, materials that normally do not convert or coalesce to impermeable films at low temperature or without fusing, but which are expected to have particularly desirable protective properties, including water resistance and fingerprint resistance can be used according to the present invention.